I've used (and independently devised) the Fahrner Image Replacement technique (FIR for short, named after Todd Fahrner), but I didn't discover the correct term for it until now. It's pretty simple. Say you want a heading as an image while still preserving the stylized text the image shows in plain text form. This is the old HTML way:

That would certainly display the alt value for people using a text-only browser like the Links browser or Lynx, but search robots index alt values differently than pure HTML text. That image is a presentational feature, and should be kept in CSS. It's fairly easily done: